Problem: The arithmetic sequence $(a_i)$ is defined by the formula: $a_i = 9 - 3(i - 1)$ What is $a_{20}$, the twentieth term in the sequence?
From the given formula, we can see that the first term of the sequence is $9$ and the common difference is $-3$ To find $a_{20}$ , we can simply substitute $i = 20$ into the given formula. Therefore, the twentieth term is equal to $a_{20} = 9 - 3 (20 - 1) = -48$.